Quantum Chemistry Guide to PTRMS Studies of As-Yet Undetected Products of the Bromine-Atom Initiated Oxidation of Gaseous Elemental Mercury

Quantum Chemical Investigation of Snow-Mercury Interactions and Their Implication of Mercury Deposition in the Arctic

Elemental gaseous Hg is emitted into the atmosphere through various anthropogenic and natural processes. Mercury's different species and respective transport ranges, atmospheric physical and chemical transformations, and interaction with the earth's surfaces all contribute to the global cycling of toxic mercury. Under sunlight, halogens, ozone, and nitro species oxidize the emitted elemental Hg to gaseous Hg (II) molecules, which deposit onto the snow and ice surfaces in the Arctic. To investigate the fate of deposited mercury, a quantum chemical investigation was conducted using first-principles density functional theory (DFT) to analyze the interaction between various mercury molecules and snow clusters of differing sizes. Results show that all oxidized mercury molecules: XHgY, BrHgOX, BrHgXO XHgOH, XHgO₂H, and XHgNO₂, with X, Y = Cl, Br, and I atoms have thermodynamically stable interactions with snow clusters. Further, the adsorption energy of all mercury molecules increases with increasing size of snow clusters. Additionally, the orientations of deposited mercury molecules on the cluster surface also influence the mercury-snow interactions.

The reaction between HgBr and O3: kinetic study and atmospheric implications

The rate constants of many reactions currently considered to be important in the atmospheric chemistry of mercury remain to be measured in the laboratory. Here we report the first experimental determination of the rate constant of the gas-phase reaction between the HgBr radical and ozone, for which a value at room temperature of k(HgBr + O₃) = (7.5 ± 0.6) × 10^-11 cm³ molecule s^-1 (1σ) has been obtained. The rate constants of two reduction side reactions were concurrently determined: k(HgBr + O) = (5.3 ± 0.4) × 10^-11 cm³ molecule s^-1 and k(HgBrO + O) = (9.1 ± 0.6) × 10^-11 cm³ molecule s^-1. The value of k(HgBr + O₃) is slightly lower than the collision number, confirming the absence of a significant energy barrier. Considering the abundance of ozone in the troposphere, our experimental rate constant supports recent modelling results suggesting that the main atmospheric fate of HgBr is reaction with ozone to form BrHgO.

Ab initio quantum-chemical computations of the absorption cross sections of HgX2 and HgXY (X, Y = Cl, Br, and I): molecules of interest in the Earth's atmosphere

The electronic-structure properties of the low-lying electronic states and the absorption cross sections of mercury halides have been determined within the UV-vis spectrum range (170 nm ≤ λ_photon ≤ 600 nm).

First kinetic study of the atmospherically important reactions BrHg˙ + NO2 and BrHg˙ + HOO

We use computational chemistry to determine the rate constants and product yields for the reactions of BrHg˙ with the atmospherically abundant radicals NO₂ and HOO. The reactants, products, and well-defined transition states are characterized using CCSD(T) with large basis sets. The potential energy profiles for the barrierless addition of HOO and NO₂ to BrHg˙ are characterized using CASPT2 and RHF-CCSDT, and the rate constants are computed as a function of temperature and pressure using variational transition state theory and master equation simulations. The calculated rate constant for the addition of NO₂ to BrHg˙ is larger than that for the addition of HOO by a factor of up to two under atmospheric conditions. For the reaction of HOO with BrHg˙ the addition reaction entirely dominates competing HOO + BrHg˙ reaction channels. The addition of NO₂ to BrHg˙ initially produces both BrHgNO₂ and BrHgONO, but after a few seconds under atmospheric conditions the sole product is syn-BrHgONO. A previously unsuspected reaction channel for BrHg˙ + NO₂ competes with the addition to yield Hg + BrNO₂. This reaction reduces the mercury oxidation state in BrHg˙ from Hg(i) to Hg(0) and slows the atmospheric oxidation of Hg(0). While the rate constant for this reduction channel is not well-constrained by the present calculations, it may be as much as 18% as large as the oxidation channel under some atmospheric conditions. As no experimental kinetic or product yield data are available for the reactions studied here, this work will provide guidance for atmospheric modelers and experimental kineticists.